Teacher Background

Each of the billions of neurons in the nervous system communicate with one another and with other cells, such as muscle cells, through special junctions known as synapses. Some neurons share synapses with thousands of other cells. Others connect with only a few cells. As noted earlier, nervous system signals travel along the cell membranes of individual neurons, but what happens at the ends of neurons? How does a signal move across the synapse to other neurons? The answers to these questions involve highly efficient mechanisms that allow signals to be transmitted from neuron to neuron.

Nervous system signals generally travel in only one direction along a neuron. Signals are received on dendrites or on the cell body and trigger an electrical impulse that moves along the axon. Once the signal reaches the end of the axon (or axon terminal) of a neuron, it must move through the synapse to the next neuron. At the most common type of synapse, known as a chemical synapse, the impulse triggers the release of chemical messengers, called neurotransmitters, from special pockets known as vesicles. Neurotransmitters released from the vesicles leave the cell and physically move through the narrow watery space (the synaptic cleft) between neurons. The space between neurons is about 20 nanometers (one nanometer equals 0.000,000,1 centimeters). Once on the other side of the gap, the neurotransmitters attach to special receptor molecules on a dendrite or on the cell body of the receiving neuron. The joining of the neurotransmitters to their specific receptor sites can promote the generation of a new electrical impulse (the neuron “fires”) OR the neurotransmitters can have an inhibitory effect, making it harder for the neuron to fire.

Biologists have identified more than 100 different neurotransmitters. Each has a different three-dimensional shape, which fits only a certain kind of receptor site. The relationship between a neurotransmitter and its receptor is similar to that of a key and a lock.

The story does not end, however, with the binding of the chemical messengers to receptors on the next neuron. If the messengers remained in place, no new signals could be received. Thus, mechanisms to remove the messenger from the synapse also exist.

In some cases, neurotransmitters simply float (diffuse) away from the synapse. Other neurotransmitters are broken down or degraded by enzymes found within the synaptic cleft. Many neurotransmitters are transported, whole, back into the neuron that released them. Some drugs, such as cocaine and fluoxetine (Prozac®), exert their effects by interfering with the removal of neurotransmitters from the synapse.

Sometimes, neurons do not communicate through neurotransmitters. Instead, an electrical charge passes directly from neuron to neuron through what is known as an electrical synapse. This type of signaling,in which the communicating neurons are very close together, is very fast and allows many interconnected neurons to fire at the same time. Electrical synapses are less common than chemical synapses, but they are very important for the normal development and function of the nervous system.

Objectives and Standards

Concepts

Nervous system signals are transmitted electrically along individual neurons.

Neurons are separated from each other by a tiny gap.

Chemical messengers called neurotransmitters cross the gap between neurons.

Neurotransmitters fit into special receptor sites on receiving neurons.

Science, Health and Math Skills

Sequencing

Communicating

Applying knowledge

Identifying patterns

Materials and Setup

Teacher Materials

24 sheets of 8-1/2 in. x 11 in. white card stock

6 bags, resealable plastic (approx. 4 in. x 6 in.)

Document projector (or overhead projector and transparency of the “Transmitters & Receivers” page from the activity, “What Is a Neuron?”)

Optional: Set of dominoes or small wooden blocks

Materials per Student

Copy of “Locks & Keys Cards” and “Rules of Play” pages

Pair of scissors

Setup

Make photocopies of the “Locks & Keys Cards” page using white card stock (four per group) and photocopies of “Rules of Play” page using copy paper (one per student).

Have students conduct this activity in groups of four.

Procedure and Extensions

Learning about chemical messengers

Remind students of the “Neural Network Signals” activity by asking, What happened when you tested whether salt water would conduct electricity? Students should remember that salt dissolved in water carried the electrical current from one foil strip to the other, thus completing the circuit.

Tell students that rapid movements of dissolved substances like those in salt also make it possible for neurons to transmit electrical signals along the lengths of their axons. In the case of neurons, a single pulse of electricity is transmitted along the axon rather than a current.

Optional: You may want to set up a row of dominoes or small wooden blocks to demonstrate how toppling one domino will set off an impulse that topples each domino in sequence, and correlate this action to the movement of an electrical impulse along a cell membrane.

Project the “Transmitters & Receivers” page. Point to the top neuron and ask, Where would signals be received on this neuron? (dendrites or cell body). If a signal travels along this neuron, where will it go? (signal will travel the length of the axon).

Point to the gap between the two neurons and ask, What happens when the signal reaches the end of the axon? How could the message get to the next cell? Allow students to discuss different scenarios. List their suggestions on the board. (You may want to group their suggestions into two broad categories: one representing scenarios related to electrical transmission and the other related to possible kinds of chemical transmissions.)

Use questions to help students evaluate their list of possible ways for signals to cross the synapse from one neuron to the next. Ask, Which of these choices would allow for rapid communication? (electrical-type communications). Which might allow neurons to send and receive different messages? (systems that use different messengers, such as chemicals).

Point out that in some cases, neurons in the human nervous system transmit messages electrically to other neurons. However, in most cases, special chemical messengers (neurotransmitters) are released and travel across the gap to the next neuron, where they attach to molecules called receptors.

Distribute photocopies of the “Locks & Keys Cards” and “Rules of Play” pages to each group of students. Have students cut out the cards and arrange one set of cards in a logical sequence using the text at the bottom of each card as a guide. Discuss the sequence of events shown in the cards with the class. Point out that even though the cards depict a sequence in which a neurotransmitter promotes the firing of another neuron, neurotransmitters also can communicate a “stop” message, which makes it harder for the next neuron to fire.

Make a list on the board of the transmission sequence in neuron communication: 1) Message Received; 2) Neuron Fires!; 3) Axon; 4) Neurotransmitters; 5) Synapse; 6) Receptor; 7) New Message; and 8) Recycle. List the sequence in order (top to bottom), but do not number the list.

Playing the game

Leave the list of steps on the board to help students as they play the game. Depending on the ages and prior knowledge of your students, you may want to erase the sequence after students have played a few rounds of “Locks & Keys.”

Explain game rules to students, which are similar to the card game “Go Fish.”

Have students play the game for two or more rounds, or until they are comfortable with the sequence of events depicted on the cards.